Alolan Vulpix can be found at Tapu Village and Mount Lanakila
Edit: However, after I looked at the Pokemon DB location guides for both Mount Lanakila and Tapu Village, it seems that it is a version exclusive, meaning that it can only be found in Pokémon Sun, not in Moon. However, at this location, you can also find another version exclusive, for Pokémon Moon this time, Alolan Sandshrew!
Also, if you're interested in evolving Alolan Vulpix into Alolan Ninetales, just use an Ice Stone!
Hope I helped!